| Drying is an operation that is an essential part of a large number of industries,which ranges from the food industry to the process industry.The methods currently in use for drying consumes a large amount of energy.Therefore,the use of existing technical means to reduce the energy consumption of drying operations in energy conservation and emission reduction strategy is of great significance.As a composite drying system,the solar-assisted heat pump drying system has the advantages of solar drying and heat pump drying.According to environmental conditions,it can be switched to an energy-efficient mode.Based on previous research,a solar-assisted heat pump drying system is proposed,and a scale down the experimental setup of a solar-assisted heat pump drying device is designed and built according to the system.A preliminary study and simulation analysis is carried out to evaluate the performance of the device.According to the proposed system,the design calculations of the system are carried out.Based on calculation results,various components are selected,and the experimental device is constructed.The system is mainly composed of a DC variable frequency compressor rated at 420 W,TEN2 type thermal expansion valve,1.2 m~2 bare-panel solar-evaporator collector,2.0 m~2 flat-plate solar air collector and a condenser.The error analysis of the test system is carried out,and the construction of the experimental setup is completed.Under different environmental conditions,the performance of the solar-assisted heat pump drying device is studied.The results show that the system performs well in the solar-assisted heat-pump drying operation mode.In solar drying mode,the average heat-collecting efficiency of the air collector is about0.562,and the Specific Moisture Extraction Rate(SMER)is up to 3.56 kg/k Wh.In the heat pump drying mode,the effects of air temperature,airspeed,and load on the system were studied by using the control variable method.The results show that as the temperature of drying air rises,the rate of drying of mushrooms will also increase,the evaporation temperature and condensation temperature of the heat pump will also rise.The average Co-efficient of Performance COP will decrease.With the increase of drying airspeed,the rate of drying of mushrooms increases slightly,the condensation and evaporation temperature of the heat pump increase,and the average COP is also increased.As the load in the drying chamber increases,the rate of drying of mushrooms decreases slightly,the evaporation and condensation temperature of heat pup remain unchanged,and the COP falls slightly.When the system was operated on solar assisted heat pump drying mode,by making use of the solar air collector only,the average temperature of inlet air of drying chamber in open-loop,semi-open loop,and closed-loop modes was59.93℃,60.58℃ and 61.95℃ respectively and the COPs were 3.25,3.14 and 2.79 respectively,which is significantly larger than the heat pump drying mode.In the later stages in the experiment,the system can be shifted to closed-loop solar assisted heat pump drying mode,which is assisted by evaporator-collector only.It can make this mode of drying reach the energy efficiency of 37.96%as compared to the closed-loop heat pump drying mode of the same conditions.To study the performance of the system for an extended period and to study the influencing factors in detail,a mathematical model was developed on MATLAB,which is verified by experimental data.For the heat pump drying mode,the results indicate that by the increase of compressor speed,the inlet air temperature of the drying chamber increases,but the COP decreases.By increasing the refrigerant charge in a specific range,the COP increases.If we fix the value for the air inlet temperature of the drying chamber,with the solar radiation intensity or ambient temperature,the COP of the heat pump increases with the fall in the compressor rotation speed. |
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